The proposed experiments will continue a series of studies intended to examine all of the ascending spinal systems thought to be importantly involved in transmitting nociceptive information in the rat. In the initial experiments, I will use electrophysiological techniques to describe the response characteristics of the cells of origin of the rat spinocervical tract (SCT). Interest in these studies derives from the known importance of the SCT in pain perception in the cat and from its recent discovery and anatomical description in the rat. I will then use anatomical techniques to determine whether the rat possesses a system of dorsal horn neurons that project to dorsal column nuclei. If such cells are shown to exist, these studies will determine their number, their location within the dorsal horn and the funiculi in which their axons ascend. Again, if such cells do exist, their response properties will be determined using physiological techniques. In a third series of studies, the transmission of nociceptive information from the thalamus to the cortex will be examined. Recent evidence has indicated that the primate ventral posterior lateral (VPL) nucleus receives nociceptive information and projects such information upon SI cortex. These findings will be pursued by using antidromic activation techniques to determine whether the previously reported nociceptive cells within the rat homologue of the VPL (ventrobasal complex) project to SI and/or SII cortex. If so, I will intraaxonally record from and physiologically identify thalamocortical axons subjacent to the cortical gray matter. Following complete physiological characterization, the axons will be iontophoretically injected with horseradish peroxidase. These studies will expand our knowledge of the processing of information regarding noxious stimuli at thalamic levels, describe the morphology of physiologically identified thalamocortical axons and determine the cortical areas that receive nociceptive input from the thalamus.